Obesity increases the risk for chronic diseases such as type 2 diabetes, cardiovascular disease, and cancer. Obesity results from the imbalance between energy intake and energy expenditure, where excess energy is stored in adipocytes as triglycerides. PPAR? is a critical regulator of adipogenesis, regulating the expression of genes that are characteristic of the adipocyte lineage. The mechanisms that facilitate PPAR?- dependent gene expression during the course of adipogenesis are incompletely understood. Previously we reported that TLE3 is a transcriptional coregulator of PPAR? and is involved in a feed-forward transcriptional program to drive adipogenesis. In the proposed studies I will test the hypothesis that TLE3 is a key determinant in driving white versus brown fat selective gene expression. Preliminary data indicates that mice overexpressing TLE3 in brown adipose tissue (BAT) have a phenotypic switch from brown to white adipose tissue (WAT). As a result, TLE3 transgenic mice have an impaired thermogenic response when challenged with cold exposure. Mechanistic studies suggest that TLE3 counters Prdm16, a transcriptional coregulator of brown fat gene expression. In specific Aim 1 I will use in vitro models to investigate the function and mechanism of action of TLE3 in executing the WAT and BAT transcriptional programs. I will utilize in vitro gain and loss of function studies to determine whether TLE3 affects white versus brown fat gene expression. Gain of function approaches will include retroviral and adenoviral expression of TLE3 and/or Prdm16. Loss of function studies will utilize white and brown TLE3F/F preadipocytes infected with control or Cre-adenovirus to generate in vitro knockouts. I will explore the mechanism of action by examining the ability of TLE3 to direct chromatin remodeling and recruitment of histone modifying enzymes to adipocyte promoters. In specific Aim 2 I will use in vivo models to define the function of TLE3 in white and brown adipose tissue and systemic lipid metabolism. I have already generated transgenic animals expressing TLE3 in adipocytes, as well as mice with conditional deletion of TLE3 in adipocytes. I will use these models to examine the ability of TLE3 to affect adipocyte gene expression, thermogenesis and lipid and glucose metabolism. The proposed studies will be completed in the laboratory of Dr. Peter Tontonoz at UCLA, who has provided an enriching environment that will facilitate the transition to an independent investigator position. The career development award will provide protected time to develop critical skills in writing, networking, and communication. In addition to attending seminars at UCLA and attending conferences, I will take courses that will enhance technical training in the use of radioisotopes for the study of metabolic pathways, protein purification and characterization, and next generation sequencing technologies for the study of epigenetics. The proposed studies are a logical transition from my postdoctoral studies in adipogenesis to the burgeoning field of brown adipocyte biology.